Computers Tools for an Information Age Chapter 6 Storage and Multimedia The Facts and More Objectives List the benefits of secondary storage Identify and describe storage media that are available for personal computers Differentiate among the principal types of secondary storage Describe how data is stored on a disk Discuss the benefits of multimedia Explain how data is organized accessed and processed 11 23 2003 2 Secondary Storage Separate from the computer itself Software and data stored on a semipermanent basis 11 23 2003 Unlike memory not lost when power is lost 3 Benefit Space Store a roomful of data on disks smaller than the size of a breadbox 11 23 2003 Diskette contains equivalent of 500 printed pages Optical disk can hold equivalent of 500 books 4 Benefit Reliability Data in secondary storage is relatively safe 11 23 2003 Secondary storage is highly reliable More difficult for untrained people to tamper with data stored on disk 5 Benefit Convenience Authorized users can easily and quickly locate data stored on the computer 11 23 2003 6 Benefit Economy Several factors create significant savings in storage costs 11 23 2003 Less expensive to store data on disks than to buy and house filing cabinets Reliable and safe data is less expensive to maintain Greater speed and convenience in filing and retrieving data 7 Magnetic Disk Storage Data represented as magnetized spots on surface of spinning disk Spots on disk converted to electrical impulses Primary types 11 23 2003 Diskettes Hard Disks 8 Diskettes Made of flexible Mylar and coated with iron oxide Has protection of rigid plastic jacket 3 diskette holds 1 44 MB of data High capacity variations 11 23 2003 Sony s HiFD holds 200 MB Imation s SuperDisk available in 120 and 240 MB versions Iomega s Zip drive available in 100 250 and 750 MB versions 9 Hard Disks Rigid platter coated with magnetic oxide Several can be combined into a disk pack Disk drive a device that allows data to be read from or written to a disk 11 23 2003 Disk drive for personal computers contained within computer housing Large computer systems may have several external disk drives 10 Reading Writing Data Access arm moves read write head over particular location Read write head hovers a few millionths of an inch above platter 11 23 2003 If head touches platter a head crash occurs and data is destroyed Data can be destroyed if head touches miniscule foreign matter on surface of disk 11 Disk Packs Each platter has its own access arm with read write head Most disk packs combine platters access arms and read write head 11 23 2003 12 Hard Disks for Personal Computers Sealed modules that mount in a 3 bay Capacity in gigabytes Accessing files much faster than accessing files on diskettes Some contain removable cartridges 11 23 2003 Iomega s Jaz drive is very popular 13 Redundant Array of Independent Disks RAID A group of disks that work together as one Raid level 0 spreads data from a single file over several drives Raid level 1 duplicates data on several drives 11 23 2003 Called data striping Increases performance Called disk mirroring Increases fault tolerance 14 How Data Is Organized Track Sector Cluster Cylinder 11 23 2003 15 Track The circular portion of the disk surface that passes under the read write head 11 23 2003 Floppy diskette has 80 tracks on each surface Hard disk may have 1 000 or more tracks on each surface of each platter 16 Sector Each track is divided into sectors that hold a fixed number of bytes Typically 512 bytes per sector Zone recording assigns more sectors to tracks in outer zones than those in inner zones 11 23 2003 Uses storage space more fully 17 Cluster A fixed number of adjacent sectors that are treated as a unit of storage 11 23 2003 Typically two to eight sectors depending on the operating system 18 Cylinder The track on each surface that is beneath the read write head at a given position of the read write heads 11 23 2003 When file is larger than the capacity of a single track operating system will store it in tracks within the same cylinder 19 Disk Access Speed Access time the time needed to access data on disk Three factors Seek time Head switching Rotational delay Once data found next step is data transfer 11 23 2003 20 Seek Time The time it takes the access arm to get into position over a particular track 11 23 2003 All access arms move as a unit All simultaneously in position over a set of tracks that make up a cylinder 21 Head Switching The activation of a particular read write head over a particular track 11 23 2003 All access arms move together but only one read write head can operate at any one time 22 Rotational Delay The time it takes for the desired data on the track to rotate underneath the read write head 11 23 2003 On average half the time for a complete revolution of the disk 23 Data Transfer The process of transferring data between its location on the disk track and memory Measures of performance Average access time Data transfer rate how fast data can be transferred once it has been found 11 23 2003 About 10 milliseconds 10 thousands of a second Can be improved by disk caching Stated in terms of megabytes per second 24 Disk Caching Disk cache a special area of memory When disk drive reads data from disk it reads adjacent data and stores it in memory When next read instruction is issued drive checks first to see if desired data is in disk cache Similar to memory caching discussed in Chapter 4 11 23 2003 25 Optical Disk Storage Provides inexpensive and compact storage with greater capacity Laser scans disk and picks up light reflections from disk surface Categorized by read write capability 11 23 2003 Read only media user can read from but not write to disk Write once read many WORM user can write to disk once Magneto optical combines magnetic and optical capabilities 26 Compact Disks CD ROM drive can only read data from CDs CD R drive can write to disk once CD ROM stores up to 700 MB per disk Primary medium for software distribution Disk can be read by CD ROM or CD R drive CD RW drive can erase and record over data multiple times 11 23 2003 Some compatibility problems trying to read CD RW disks on CD ROM drives 27 Digital Versatile Disk DVD Short wavelength laser can read densely packed spots DVD drive can read CD ROMs Capacity up to 17GB Allows for full length movies Sound is better than on audio CDs Several versions of writable and rewritable DVDs exist 11 23 2003 28 Multimedia Presents information with text